A known electric control valve for a coolant compressor in a motor vehicle, for example as disclosed in DE 10 2011 117 354 A1, controls the coolant flow from a high-pressure region into a crankcase-chamber-pressure region of the coolant compressor. In a crankcase of a coolant compressor, a plurality of pistons are arranged so as to pump coolant into a high-pressure chamber. The movement of the pistons is guided by a rotating wobble plate.
If the wobble plate, which rotates by a belt drive, has a tilt angle other than zero, this leads to an axial stroke movement of the pistons during a rotation of the wobble plate. Coolant is thus sucked up from the suction chamber of the coolant compressor and pumped into the pressure chamber. The suction chamber is connected to the connector of the coolant compressor that is on the suction pressure side, and this connector is in turn connected, when mounted in the motor vehicle, to the suction-pressure region of the air-conditioning system; in particular to the output of the evaporator. The pressure chamber is connected to the output of the coolant compressor that is on the high pressure side, and this output is in turn connected to the input of the evaporator by way of the high-pressure region of the air-conditioning system, in particular via a heat exchanger (condenser) and an expansion valve. To adjust the delivery volume and control the coolant flow, it is known to vary the tilt angle of the wobble plate in the coolant compressor. If, for example, the coolant compressor is pre-set for a maximum delivery volume, pivoting the wobble plate back brings about a decrease in the axial stroke movement of the pistons of the coolant compressor and thus a reduction in the delivery volume of coolant.
It is further known to undertake this type of control of the coolant flow using a control valve. In this case, the coolant flow between the high-pressure region and the crankcase-chamber-pressure region is controlled by way of the control valve. The control valve has three connectors in the valve housing which are connected respectively to the high-pressure region, the suction-pressure region, and the crankcase-chamber-pressure region of the coolant compressor. The control valve controls the coolant flow between the high-pressure region and the crankcase-chamber-pressure region.
If, for example, in one position the control valve opens the connection between the high-pressure region and the crankcase-chamber-pressure region of the coolant compressor, coolant flows through the control valve from the high-pressure region into the crankcase-chamber-pressure region; this results in a rise in pressure in the crankcase-chamber-pressure region. If in a further position the control valve closes the connection between the high-pressure region and the crankcase-chamber-pressure region of the coolant compressor, coolant flows through the permanently open passage provided in the coolant compressor from the crankcase-chamber-pressure region into the suction-pressure region; this results in a fall in pressure in the crankcase-chamber-pressure region. As a result of the rise in pressure brought about by the control valve in the crankcase-chamber-pressure region, the wobble plate is caused to pivot back. This decreases the axial stroke movement of the pistons of the coolant compressor and the delivery volume of the coolant compressor is reduced. As a result, the pressure in the high-pressure region of the air-conditioning system does not increase any further.
As a result of the fall in pressure brought about by the control valve in the crankcase-chamber-pressure region, the wobble plate is caused to pivot out (in other words to tilt). This increases the axial stroke movement of the pistons of the coolant compressor and the delivery volume of the coolant compressor is increased. As a result, the pressure in the high-pressure region of the air-conditioning system increases further. Usually, the wobble plate is held in a tilted initial position by spring tension, in such a way that, in the event of a subsequent fall in pressure in the crankcase-chamber-pressure region, the wobble plate pivots back into the initial position and ensures an initial setting for the delivery volume in the coolant compressor.
Conventionally, a protection mechanism is integrated into a coolant compressor and prevents the coolant evaporator from icing up, which would reduce or prevent the airflow into the passenger compartment. The coolant evaporator starts to ice up as soon as the suction pressure falls below a particular pressure. An example implementation of a protection mechanism of this type includes a bellows, made of metal, integrated into the control valve, and arranged in the control valve in such a way that it can throttle the coolant compressor down. For this purpose, the bellows is filled with a gas mixture at a particular pressure.
If the pressure prevailing in the suction-pressure region of the control valve falls substantially below the fill pressure of the bellows, the volume of the gas mixture in the bellows increases in relative terms. As a result of the construction, the bellows then unfolds in a concertina shape and accordingly becomes longer. Conversely, if the pressure prevailing in the suction-pressure region of the control valve substantially exceeds the fill pressure of the bellows, the volume of the gas mixture in the bellows decreases in relative terms. As a result of the construction, the bellows then folds up in a concertina shape and accordingly becomes shorter.
This mode of operation of the bellows is exploited by the safety mechanism for the control valve in that the bellows cooperates with the valve body in such a way that, if there is a fall below a critical pressure in the suction-pressure region, it mechanically transfers the valve body into the position in which the coolant compressor is throttled down. During the construction of a control valve, the fill pressure and the type of gas mixture in the bellows are specifically selected in such a way that, if there is a fall below a minimum pressure in the suction-pressure region of the control valve, the bellows moves the valve body into the position in which the high-pressure region is connected to the crankcase-chamber-pressure region.
The rise in pressure in the crankcase-chamber-pressure region, brought about by the bellows, causes the wobble plate to pivot back. As a result, the axial stroke movement of the pistons of the coolant compressor and the delivery volume of the coolant compressor are reduced and the coolant compressor is throttled down. As a result, the pressure in the suction region of the coolant compressor does not fall below the limit value and the coolant evaporator is prevented from icing up.
A bellows of this type, however, is a mechanically operating component which has a slow response time as a result of the construction and also deteriorates with age. Thus, for example, frequent unfolding in a concertina shape and subsequent folding up of the bellows results in material fatigue therein. Further, under some circumstances, it cannot be ensured that the bellows filled with a gas mixture will remain tight throughout its service life.